Thanks capedreamer, but I'm not jostling for the last word. These are mine, but others are welcome to follow.
I stated I don't condone adults self-upgrading, but asked the forum if it's a different scenario with a small boy sitting next to his father in an unoccupied seat after the meal had been served. Most reactions were themselves interesting, revealing and very linear. Many people used some slippery logic and all voiced a strong opinion. I believe the comment above about 'quoting poetry' was an (embarrassed?) reaction to what may have been my stinging observation: that the most trivial matters are often those most heatedly opposed.
I think that the SD did a good thing by asking the child to return to his original seat. But I would not have raised an eyebrow in protest had the kid been allowed to stay. It appears my tolerance level is in the minority amongst this crowd.
Long may they fly in blessed, non-discretionary exclusivity.